Russia has accused Britain of preparing an "official witch-hunt" against president Vladimir Putin's administration amid "panic and hysterics" over a loss of control in Europe.
The Russian embassy to Britain posted online an extraordinary attack on the Government, accusing it of planning to brief Donald Trump's incoming United States administration against Moscow.
It comes as Prime Minister Theresa May prepares for her first meeting with the US president-elect, expected in the spring, and Foreign Secretary Boris Johnson said Russia was "up to all sorts of very dirty tricks" such as cyber warfare.
The embassy also speculated that the Government was seeking to rerun the Brexit referendum by claiming it was tainted by Russian influence and attacked the British intelligence services.
It directly criticised former MI6 chief Sir Richard Dearlove for standing down from a Cambridge University intelligence forum over fears of Kremlin influence.
The embassy also suggested the UK was being hypocritical when criticising alleged Russian war crimes in Syria, highlighting the use of British-made banned cluster bombs in the civil war in Yemen by a Saudi-led coalition.
And it claimed Britain had once again "outsourced" the fight against extremism to Russia.
"Ultimately, defeating Nazi Germany was 'outsourced' to the Soviet Union," the post said.
"In Syria, overlooking Isis and then 'Nusra' (al-Nusra Front), both proscribed by the UN, means pretty much the same."
It added: "We don't expect HMG (Her Majesty's Government) to win this argument in an open and reasoned debate. We also think that it is plainly wrong for one UNSC (UN Security Council) permanent member to brief against another. And we challenge the mainstream British media to publish this comment."